Addition, during TiO2 surface treatment, of certain chemicals under specific conditions significantly enhance a number of pigment performance properties in paints. These properties include durability and ease-of dispersion. The chemicals added during surface treatment that improve pigment end-use performance include metal oxides, particularly aluminum oxides and silicon dioxide. The exact nature of these oxides may in turn be influenced by the conditions under which they are formed and the presence of other reagents during the precipitation process. The present invention provides an easy dispersing, high durability TiO2 pigment and method of making that pigment. Such super-durable TiO2 pigment with improved gloss retention (durability) and dispersibility (easy dispersing) is useful in a variety of exterior applications, including but not limited to powder, coil, automotive.
Some multi-purpose TiO2 pigment grades that are commercially available are treated with a coating of amorphous alumina. In these products there is no silica present as a surface treatment. Although these products have durability that is sufficient for architectural exterior coatings, they are not super-durable. In these products materials such as zirconia and tin are used to stabilize the amorphous alumina. The use of zirconia and tin makes these grades more expensive to manufacture and to use. The present invention uses citric acid to stabilize the amorphous alumina. The combination of silica and citric acid stabilized amorphous alumina on a pigment seems to be a new approach that provides performance and cost efficiencies.
U.S. Pat. No. 3,825,438 teaches a process to make a coated titanium dioxide pigment with one or more hydrous oxides in the presence of a polyhydric alcohol and/or a carboxylic acid. In this process, the alcohol or acid may be added at any time in the process prior to the completion of the deposition of the coating. The teachings in U.S. Pat. No. 3,825,438 solve the problem of pigment agglomeration on storage. A broad series of organic additives are taught in this patent to accomplish this desired result. A critical teaching is that the point at which the alcohol and/or acid are added in the process has no influence on the results achieved in the pigment product.
U.S. Pat. No. 5,340,393 teaches a method of making a non-agglomerated water insoluble inorganic particle having a dense, amorphous silica coating. Citric acid is identified as a possible optional charge stabilization dispersing acid that may be used in the process to prevent agglomeration of particles in the process. This patent offers no teaching as to the deposit of amorphous alumina or special properties resulting from the use of citric acid over other dispersants taught.
U.S. Pat. No. 5,730,796 teaches a process to make a durable pigment having a coating of cerium oxide, a coating of dense, amorphous silica and preferably an outer coating of crystalline (hydrous) alumina. It makes no teaching or suggestion with respect to the use of citric acid in deposition of silica or aluminia to enhance pigment properties
U.S. Pat. No. 5,700,318 teaches an alumina coated inorganic pigment having a first coating of boehmite alumina, a second coating of amorphous alumina and a third coating of boehmite alumina. Adjustment of precipitation pH is taught as a means to produce the crystalline and amorphous alumina coatings. No teaching or suggestion is made with respect to the use of citric acid in this process.
U.S. Pat. No. 3,506,466 teaches the use of a salt of a water soluble alcohol amine and an easily water soluble oxycarboxylic acid to enhance dispersibility. The salt is added after wet treatment and before grinding the pigment.
U.S. Pat. No. 5,824,146 teaches a titanium dioxide pigment with amorphous alumina product in vapor phase oxidation of titanium tetrachloride in the presence of aluminum trichloride. The pigment is slurried using a citric acid dispersant.
U.S. Pat. No. 5,824,145 teaches mixtures of titanium dioxide pigment slurry and slurry of amorphous alumina hydroxide formed from mixing the two component slurries and using citric acid as a dispersing agent foe the mixed slurries. There is no wet treatment process disclosed in this patent.
U.S. Pat. Nos. 2,885,366 and 3,437,502 provide the basic teachings for making a durable titanium dioxide pigment having an amorphous silica coating followed by a coating of mixed crystalline alumina. Neither of these patents teach the use of citric acid during the wet treatment process used to deposit these treatments on the pigment surface.